Problems with fogging have plagued the photographic industry from its inception. Fog is a deposit of silver or dye that is not directly related to the image-forming exposure, i.e., when a developer acts upon an emulsion layer, some reduced silver is formed in areas that have not been exposed to light. Fog can be defined as a developed density that is not associated with the action of the image-forming exposure, and is usually expressed as "D-min", the density obtained in the unexposed portions of the emulsion. Density, as normally measured, includes both that produced by fog and that produced as a function of exposure to light. It is known in the art that the appearance of photographic fog related to intentional or unintentional reduction of silver ion (reduction sensitization) can occur during many stages of preparation of the photographic element including silver halide emulsion preparation, spectral/chemical sensitization of the silver halide emulsion, melting and holding of the liquid silver halide emulsion melts, subsequent coating of silver halide emulsions, and prolonged natural and artificial aging of coated silver halide emulsions.
The chemicals used for preventing fog growth as a result of aging or storage are generally known as emulsion stabilizers. Prolonged storage, heat and high humidity often exacerbate such fog growth. Emulsion or raw stock stabilization is often referred to as storage stability or raw stock keeping (RSK).
The control of fog, whether occurring during the formation of the light-sensitive silver halide emulsion, during the spectral/chemical sensitization of those emulsions, during the preparation of silver halide compositions prior to coating on an appropriate support, or during the aging of such coated silver halide compositions, has been attempted by a variety of means. Mercury-containing compounds, such as those described in U.S. Pat. Nos. 2,728,663; 2,728,664; and 2,728,665, have been used as additives to control fog. Thiosulfonates and thiosulfonate esters, such as those described in U.S. Pat. Nos. 2,440,206; 2,934,198; 3,047,393; and 4,960,689, have also been employed. Organic dichalcogenides, for example, the disulfide compounds described in U.S. Pat. Nos. 1,962,133; 2,465,149; 2,756,145; 2,935,404; 3,184,313; 3,318,701; 3,409,437; 3,447,925; 4,243,748; 4,463,082; and 4,788,132 have been used not only to prevent formation of fog but also as desensitizers and as agents in processing baths and as additives in diffusion transfer systems. Organic compounds having a polysulfur linkage comprised of three or more sulfur atoms, and organic compounds having a heterocyclic ring having at least two thioether linkages or at least one disulfur linkage, such as those described in U.S. Pat. No. 5,116,723, in combination with nitrogen-containing cyclic compounds have also been discussed as suppressing fog and improving raw stock stability.
For highly sensitive silver chloride emulsions, storage fog is of particular concern. For these silver chloride emulsions, mercapto compounds and mercury salts are especially useful. The problem with stabilizers such as mercaptans and mercury salts is that when used in sufficient quantities to stabilize fog formation after storage, such stabilizers reduce the sensitivity of the emulsion considerably. Thus it is required that a mercaptan antifoggant be protected such that no sensitivity is lost when a sufficient amount is used to stabilize the emulsion. At the same time, it is also required that during incubation, when the temperature and humidity are high, the lability of the protecting group be sufficient to allow the mercaptan group to be revealed and the stabilizing function performed. Mercury compounds have the additional disadvantage of being harmful to the environment.
Mercaptan precursors have been taught in the photographic art. Azoles which hydrolyze in alkaline solution to yield mercaptoazoles are discussed in U.S. Pat. No. 3,260,597. U.S. Pat. No. 3,615,617 discusses the use of thiocarbonic acid esters and thiocarbamates as emulsion stabilizers. U.S. Pat. No. 4,840,871 describes a number of mercaptotetrazoles in which the mercaptan group is bonded to alkali labile substituents including sulfonylethyl groups (--CH.sub.2 CH.sub.2 SO.sub.2 R). U.S. Pat. No. 4,511,644 discusses the use of an alkoxycarbonylethyl (--CH.sub.2 CH.sub.2 COOR) moiety as a mercapto protecting group in a color diffusion transfer photographic element.
Alkoxycarbonyl (--COOR) protected mercapto heterocycles have also been mentioned in the photographic art. GB 1,186,441 discusses the stabilization of developed silver images with alkoxycarbonyl mercaptans in black and white photography. U.S. Pat. No. 3,364,028 describes the use alkoxycarbonyl mercaptans as yellow fog-preventing agents in photographic materials. U.S. Pat. Nos. 3,751,249 and 3,839,041 report the use alkoxycarbonyl mercaptan compounds for stabilization of the developed image in photothermographic elements. U.S. Pat. No. 4,952,491 discusses the use of alkoxycarbonyl protected mercaptan compounds in tabular silver chloride emulsions. U.S. Pat. No. 5,478,721 describes the use of similarly protected mercaptans in non-tabular silver chloride emulsions. One problem with the alkoxycarbonylmercaptans is that the protecting group does not have sufficient lability to fully allow the antifogging activity of the mercaptan compound to be realized during storage of the photographic coatings. U.S. Pat. No. 3,791,830 discusses the use of arylsulfonylethoxycarbonyl (--COOCH.sub.2 CH.sub.2 SO.sub.2 Ar) and arylthioethoxycarbonyl (--COOCH.sub.2 CH.sub.2 SAr) protected mercaptans in silver bromoiodide emulsions in color negative films.
Despite the vast amount of effort which has gone into methods to control fog in photographic elements there is a continuing need in the industry for practical and environmentally benign stabilizers, particularly for high chloride emulsions, which do not otherwise adversely affect the performance of the photographic element.